The invention relates to an apparatus for the positioning of measuring instruments.
Such apparatuses are used, for example, in the manufacture of plastic films to position measuring instruments with respect to the material under test for the determination and monitoring of the thickness of the plastic films. Plastic films are extruded, for example, as a tube. An arcuate or annular track is arranged around the tube of plastic film, as concentric as possible thereto, just after the extruder as seen in the direction of movement of the tube. The actual measuring unit with the measuring head and adjusting arm is arranged on this track. The adjusting arm with the measuring unit and the measuring head is radially movable, for example, in the case of a circular, annular track. It is thus possible to position the measuring head radially correctly with respect to the, for example, tubular material under test.
The sensing head of the measuring head monitors the thickness of a plastic film only over a width of a few millimeters or centimeters. As a rule, the thickness of plastic films does not change suddenly and by a great deal very quickly. Differences in the thickness of films are substantially more common over the periphery of a foil extruded as a tube. Differences can therefore predominantly be seen in the thickness transverse to the direction of movement of the extruded foil, in the so-called cross-profile. It is therefore sufficient in most cases to move the measuring head, for example, periodically over the periphery of the tube in order to determine deficiencies in the thickness of the plastic film. When the measuring head moves to and fro on the annular track, it is known, for example, as reversing.
This to-and-fro movement of the measuring device over the periphery of the tube or the orbital movement around the tube can be generated in a number of different ways. For example, a carriage can be arranged on the rail-like track and the measuring unit attached thereto. The carriage can be self-propelled or driven in another way such as by a revolving rope or belt. Flexible cables and sliding contacts such as rotor slip rings can be provided for the transmission of the measuring signals, control signals and/or the power supply of the measuring unit and the drive. The measuring signals and the control signals can also be transmitted in a wireless manner.
Prior apparatuses for measuring the thickness of plastic films, e.g. of extruded polyethylene films, are made such that the track is arranged concentrically around the film tube extruded from a blow head between the blow head and a winding device. The measuring unit travels around the moving film bubble on the track. The sensing head of the measuring unit is mounted on a radially movable arm and is thus positioned exactly with respect to the film. The positioning can be necessary with a precision in the range of millimeters or fractions of millimeters, especially, but not only, when the material under test should not have any contact with any part of the sensing head, i.e. when the measurement is to be made in a non-contact process. The arm can here effectively be movable as a whole; however, it can also be a telescopically extendable and retractable arm, i.e. an arm which can be moved forwards and backwards in a radial direction, on which the sensing head of the measuring unit is attached. A linear motor or a pneumatic linear adjustment device can also be suitable. Very fast and very fine positioning in a range of a few, e.g. approximately 4, centimeters is possible with a linear motor.
In a unit with which, for example, tubular films of different diameters should be monitored, the arm to which the sensing head is attached is moved to different distances. In order to ensure the required stability, the extension region of an arm built at a reasonable cost is limited to approximately 50 cm to 85 cm. As a result, the unit is limited to use with film tubes having diameters differing in the range from approximately one to two meters.
The apparatus of the present invention constitutes a substantial improvement over the prior art and has an annular or arcuate track to which a measuring device is attached. In order to position the sensing head radially with respect to the material under test, a crossbeam is provided which supports the measuring device. Bearings are arranged between the track and the crossbeam. In the apparatus for positioning, the position of the crossbeam with the measuring device can be changed in a radial direction to the track with respect to the track.
It is easily possible with the apparatus in accordance with the invention to substantially expand the working range of the unit and to ensure the mechanical stability. If a circular track has a diameter in the order of approximately six to seven meters, the working range of the measuring unit can easily be in the range of a number of meters. That is, film tubes with diameters, for example, of approximately one to eight meters can be monitored without the accuracy of the positioning suffering. With the new apparatus in accordance with the invention, it is therefore possible to position a measuring unit, and in particular the sensing head of this measuring unit, in a mechanically stable manner over a large range.
The invention is described in more detail below with reference to the schematic drawings.